The invention relates to a semiconductor processing technique for growth of a semiconductor film on a substrate.
In accordance with the invention, a semiconductor film is grown on a substrate at temperatures below the pyrolytic threshold of a semiconductor-bearing gas. The gas is photodissociated to a species which pyrolyzes at the surface of the substrate to form several monolayers of semiconductor material which subsequently catalyze the decomposition of the gas. The stable species diffuses in the gas phase from the region illuminated by the laser to the substrate's surface. The photodissociation occurs at a point spaced above the substrate by a distance which can range from one to &gt;100 mean free paths for the collision of the photodissociated species with the background gas. In other words, the photodissociated species of the gas is stable against hundreds of collisions and lives long enough to reach the substrate. The substrate is heated to a temperature greater than the pyrolytic threshold of the photoproduced species and less than the pyrolytic threshold of the gas.
In accordance with the invention, the growth of semiconductor films by chemical vapor deposition (CVD) can be initiated ("triggered") or sustained at temperatures well below those normally necessary. The initial part of the process is photochemical and involves laser irradiation of a gas above the substrate. The laser radiation photodissociates or photoionizes a semiconductor-bearing molecule in the gas phase. The photofragments migrate to the substrate, at which point some decompose, leaving behind several monolayers. These monolayers are catalytically more active than the substrate itself. Conventional CVD then proceeds once these few monolayers are grown, even though the substrate temperature is below that normally required to initiate CVD. The laser radiation is needed only to trigger such low temperature growth process. Film growth continues after the laser is turned off.